Re: [PATCH 5/5] xdiff: NUL-terminate buffers read by read_mmfile()

[Patrick Steinhardt <ps@pks.im>]

On Tue, Sep 29, 2026 at 02:55:04AM -0400, Jeff King wrote:
> Since an mmfile_t is a ptr/len pair, our read_mmfile() allocates exactly
> the number of bytes we claim to store. But in many other places in Git,
> we add an extra NUL "just in case", which can help avoid read overruns
> due to off-by-ones or the use of string functions.
> 
> I don't know of any path that would benefit from this, but I noticed it
> while converting ll_ext_merge() to use read_mmfile(), since its original
> code did add a NUL byte (even though I cannot find any case where it
> would have mattered). Let's teach read_mmfile() to add this defensive
> NUL; it probably doesn't help anything, but nor should it hurt.
> 
> Note that the matching read_mmblob() doesn't need the same treatment.
> Its buffers already have a NUL from the object-reading code (which uses
> the same defensive trick).
> 
> As a bonus, we can get rid of the hack in read_mmfile() to handle empty
> files by allocating a single byte.
> 
> Signed-off-by: Jeff King <peff@peff.net>
> ---
> This one is obviously optional, which is why I put it last.

Hm, I'm somewhat indifferent here. It always feels a bit weird to be
this defensive because "programming errors", as the next question then
is "but what about all the other errors where we're not defensive?" But
the xdiff code is complex enough with a bunch of pointer arithmetics, so
maybe it's not even that bad of an idea.

That being said, I feel like a better course of action could be to use a
fuzzer for this code, because as far as I'm aware we have none yet, and
that would potentially shake out a bunch of bugs. But that still doesn't
really help us to catch platform-specific bugs due to different integer
sizes.

The counterargument is that before your 3/5 we used to use xmallocz, so
you're essentially just reinstating the previous safety guards.

> diff --git a/xdiff-interface.c b/xdiff-interface.c
> index bc340d5a8a..b3e9f1952b 100644
> --- a/xdiff-interface.c
> +++ b/xdiff-interface.c
> @@ -166,7 +166,7 @@ int read_mmfile(mmfile_t *ptr, const char *filename)
>  	if (!(f = fopen(filename, "rb")))
>  		return error_errno("Could not open %s", filename);
>  	sz = xsize_t(st.st_size);
> -	ptr->ptr = xmalloc(sz ? sz : 1);
> +	ptr->ptr = xmallocz(sz);

I was staring at this code a while before I noticed the added `z` at the
end of this function.

Patrick